Will a spellcaster interrupted mid-preparation have spells they did not use during the previous adventuring day?

Question

As a follow-on to this question, I pose the question "would a prepared caster (wizard/cleric/druid/paladin/ranger and their derivatives) have whatever spells they did not use the previous day if they were interrupted mid-preparation?"

Alternatively, does preparing spells "clear" out the previous day's spells at the beginning of preparation then refill the spell slots they occupied with fresh spells at the end of the process, or does it "set up" the slots to be refilled and then perform the actual transfer atomically at the end? (Based on the answers to the previous question -- I can safely say that they do not have any new spells, meaning that the idea that they'd have a mix of old and new spells if they were interrupted is off the table.)

Answer 1

Yes, spells which are already prepared beforehand are untouched by the preparation process, both arcane:

Until he prepares spells from his spellbook, the only spells a wizard has available to cast are the ones that he already had prepared from the previous day and has not yet used. […] If a wizard already has spells prepared (from the previous day) that he has not cast, she can abandon some or all of them to make room for new spells.

… and divine:

She cannot, however, abandon a previously prepared spell to replace it with another one

Interrupting the preparation process will only affect the spells being prepared, not the already-prepared spells. Note however that choosing to replace existing arcane spells at the beginning of preparation wipes those, and they will therefore not be available should the process be interrupted.